1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) device, and more particularly, to an organic EL device having a thickness of an organic film set so as to have a small difference in the luminance among red (R), green (G) and blue (B) lights depending on the change in thickness of the organic film.
2. Description of the Related Art
In general, conventional organic EL devices are of a self-emission type, and much attention has been paid thereto because they have advantageous features suitable for next generation devices, such as a wide viewing angle, a high contrast ratio and a high response speed.
EL devices are classified into inorganic EL devices and organic EL devices according to materials to form emitter layers. In particular, studies of organic EL devices have been briskly carried out because of their advantages, including good characteristics in terms of brightness, drive voltage, response speed, multi-color luminescence and so on.
An organic EL device is basically configured such that a positive electrode layer with a predetermined pattern is formed on a substrate. Then, a hole transport layer, a light emitting layer and an electron transport layer sequentially stacked on the positive electrode layer, and a negative electrode layer with a predetermined pattern is formed on the electron transport layer in a direction orthogonal to the positive electrode layer. Here, the hole transport layer, the light emitting layer and the electron transport layer are organic thin films made from organic compounds.
In the field of organic EL devices, many attempts to achieve the maximum emission efficiency and luminance have been made by controlling the thickness of the organic thin films. For example, Japanese Patent Laid-open Publication No. Hei 4-137485 discloses, in the construction consisting of a positive electrode, a hole transport layer, a light emitting layer, an electron transport layer and a negative layer sequentially stacked, a method to improve the emission efficiency by setting the thickness of the electron transport layer to be 30 to 60 nm. Also, Japanese Patent Laid-open Publication No. Hei 4-328295 discloses that the light luminance is substantially increased by mutual interference between the light generated from a light emitting layer and the light reflected from a cathode by adjusting the thickness of an electron transport layer. Japanese Patent Laid-open Publication No. Hei 7-240277 discloses an organic EL device with improved luminance, inter alia, color purity of blue emission by controlling the optical thickness of various layers.
Such organic EL devices are constructed such that optical thicknesses of the layers are varied according to color in order to increase the luminance, which is, however, quite difficult to achieve in actual mass production processes.
To solve the above problems, the optical thickness is adjusted to be suitable for a specific color that has rather poor characteristics. Thus, deterioration of the electro-optical characteristics of other colors cannot be avoided.